Eogenetic karst, glacioeustatic cave pools and anchialine environments on Mallorca Island: a discussion of coastal speleogenesis

Abstract

Paper presented during the 14th International Congress of Speleology, Kalamos (Greece) 21-28 August 2005. 21-28 AUGUST 2005 Anniversary 58 Angel Gines and Joaquin Gines apply to a wide range of coastal karst areas throughout the world. The term eogenetic karst has been introduced recently by Vacher & Mylroie (2002) on the basis of the term eogenetic stage used by Choquette & Pray (1970) in their classification of postdepositional stages affecting carbonate porosity. Eogenetic karst was defined by Vacher & Mylroie (2002) as: “...the land surface evolving on, and the pore system developing in, rocks undergoing eogenetic, meteoric diagenesis... [That is, the karst developed on] young limestones experiencing meteoric diagenesis in the vicinity of their deposition”. The term glacioeustatic cave pools was introduced by Gines & Gines (1977) to describe the unusual aquatic environments found in many coastal caves of Mallorca; glacioeustatic cave pools are considered to result from “passive drowning” of preexisting caves by a rise of the water table due to glacioeustatic sea levelchanges. These were termed sea-level pools by Palmer et al. (1977) and Gines et al. (1981a & 1981b). The term anchihaline was introduced by the carcinologist Holthuis (1973) in the oddly spelled form “anchialine” pools (sic), in reference to water in hollows near the coast that are influenced by the present sea level. However, regarding the traditional usage in ecology and natural sciences of similar terms (like halocline, halite, euryhaline, halophile, etc.), we think it should be preferable to use the alternative spelling anchihaline, as suggested by Sket (1996). Unfortunately, many recent publications seem to have standardized the term anchialine, as accepted implicitly by Sket (2005) in his recently published definition: “Anchihaline (or anchialine) habitats are water bodies in hollows along the sea coasts where the influence of the sea may be felt and which are inhabited by some subterranean species... Such a habitat may contain seawater, but it primarily has layers of different brackish salinities”. The ecological concept of anchialine habitats has been discussed and refined by Stock et al. (1986), Sket (1996, 2004 & 2005), and Iliffe (2003). An interesting earlier approach to some karstic anchialine habitats can be found in Riedl & Ozretic (1969). THE MAJORCAN COASTAL KARSTS About 45% of the 626 km of shoreline on Mallorca Island, consists of well-karstified limestones (Balaguer, 2005). Coastal karst features are significant forms in the littoral landscape and include different types of caves and karst remnants, as well as widespread solutional microforms and biokarstic karren (Gines, 2000b). In addition, the hydrogeology of much of Mallorca is strongly influenced by local patterns of interconnected porosity resulting from coastal karst development. Many karst aquifers lie in direct hydrologic contact with the sea along the coastline of the island, both in mountain ranges, such as Serra de Tramuntana Fig. 1. Geological map of Mallorca showing the location of the caves cited in the text as well as the main karstic regions of the island. International Journal of Speleology, 36 (2), 57-67. Bologna (Italy). July 2007 59 Eogenetic karst, glacioeustatic cave pools and anchialine environments on Mallorca Island and Serres de Llevant, and in the post-orogenic (late Miocene) carbonate platform called Migjorn (Fig. 1). The mountainous karst is developed preferentially on limestones and dolomites of Mesozoic age (namely Rhaetian and Lower Jurassic); their occurrence along more than 175 km of coastline includes good examples of conventional telogenetic coastal karst (i.e., formed by exposure to karstification of the carbonates after deep burial). On the other hand, the karst developed along 120 km of the coast of Migjorn differs in typical examples of eogenetic karstification. This area is characterized by nearly flat-lying littoral plateaus made of very porous limestones and calcarenites of Upper Miocene age. The Serra de Tramuntana is the highest and most important mountain range of Mallorca. It constitutes the whole northwestern side of the island and has a NE-SW alignment, its structure being characterized by a series of imbricate overthrust sheets (Gelabert et al., 1992) mainly composed of Mesozoic carbonate rocks. Impervious Upper Triassic marls (Keuper) at the base of the thrusts isolate and partition the karstic aquifers (basically Rhaetian dolomites and Lias limestones). In the slopes that delimit the 90-km-long NW shoreline of the island, widespread outcrops of Lower Jurassic (Lias) limestone produce magnificent cliffs as well as a variety of karst landforms and hydrogeologic conditions along their contact with the sea. The Serres de Llevant is a mountain ridge in eastern Mallorca with less rugged topography than Serra de Tramuntana. Owing to alpine tectonics, both mountain ranges were built between Oligocene and Middle Miocene times (Gelabert, 1998) and have thrust sheets and related folds that constitute an integrated tectonic assemblage whose transport direction was preferentially towards the NW. In the Serres de Llevant, the main mountains are characterized by cliffs of Jurassic limestones and the more rounded forms correspond to crushed dolomites, marls, and thin-bedded limestones ranging from Jurassic to Cretaceous age. Although only part of this mountain range borders the shoreline, it contains numerous interesting examples of telogenetic coastal karst, including the celebrated show-cave known as Coves d’Arta. The highly porous Upper Miocene limestones, which outcrop in Migjorn, constitute a post-orogenic platform that surrounds the “alpine structured” mountain ranges. This platform was formed by progradation of a Tortonian Reefal Complex composed of reef tracts and calcarenites overlying calcisiltites, which grades upward through mangroove facies and stromatolitic limestones into sand shoal oolites and calcarenites (Fornos et al., 2002a). The upper unit, called the Terminal Complex, is probably related to the Messinian crisis. These carbonates stretch across a relatively flat surface behind the coastal cliffs of the southern and eastern shores of the island that is interrupted only by incised dry valleys ending at littoral coves. The thickness of these Upper Miocene carbonates which onlap the alpine folded basement is very irregular, showing mean values around 70 m and occasionally exceeding 120 m. Because the Mesozoic basement consists mainly of karstifiable rocks (limestones and dolomites) with only minor marl intercalations, the karst area of Migjorn shares some resemblances with the complex island type from the Carbonate Island Karst Model, or CIKM, as postulated by Jenson et al. (2006); however, in this case the basement is not an impervious one and can even contribute to the recharge of the overlying Upper Miocene aquifers. Regarding tectonics, Mallorca is part of the folded and thrusted belt resulting from the continental collision between the African and Iberian plates (Gelabert et al., 1992). Such a collision took place from the Upper Cretaceous to the Middle Miocene. In spite of the relevance of this orogenic event, the present geological architecture of the island is the result of a complex evolution involving extensional processes -occurred from the Neogene to the Quaternarywhich are superimposed on the alpine compressive structure. The Upper Miocene carbonate platform of Migjorn was affected by such extensional tectonic activity and, in this way, the resulting coastal morphology was mostly controlled by recent normal faults which delimit the seaward border of the platform. Furthermore, this extensional post-orogenic activity is responsible for a marked tilting that affects the Migjorn plateau, being depressed towards the south and uplifted when reaching the W and NE ends of the karst area (Fornos et al., 2002b). The differential uplifting left the reefal Tortonian facies outcropping near the sea level at the southernmost part of the island whereas it appears higher than 40 m in the western and northeastern borders of Migjorn. Because of the great extent of coastal karst in Mallorca and the remarkable contrast between the telogenetic karst of the Tramuntana-Llevant mountains and the eogenetic karst of the Migjorn platform, it is worthwhile to point out the particular characteristics of this area, especially with regard to speleogenesis. Our present knowledge is limited, but further studies and new explorations are in progress. CHARACTERISTICS AND SPELEOGENESIS OF A CLASSIC MAJORCAN CAVE: COVES DEL DRAC Coves del Drac (Fig. 2) is the most celebrated cavern in Mallorca and is visited each year by more than 800.000 tourists, and in this regard is one of the most important show-caves in the world. In spite of the great number of visitors and many publications about early explorations, the literature on the geology and ecology of the cave is scanty (Gines & Gines, 1992). Explorations began in the 19th century and were especially promoted by the Austrian Archduke Ludwig Salvator Habsburg-Lothringen, a learned maecenas and natural scientist who came to Mallorca in 1867. The Archduke sponsored both the first topographic survey of the cave, carried out by Friedrich Will in 1880, and the famous exploration undertaken by Edouard-Alfred Martel in 1896. The cave map produced by Martel, Pedro Bonet de los Herreros, Fernando Moragues, and Louis Armand during their historic exploration, showed nearly twice the length of the earlier map. It is noteworthy that the 1896 extensions International Journal of Speleology, 36 (2), 57-67. Bologna (Italy). July 2007